Embryonic stem cells (ES cells) are stem cells established from human or mouse early embryos which have a characteristic feature that they can be cultured over a long period of time while maintaining pluripotent ability to differentiate into all kinds of cells existing in living bodies. Human embryonic stem cells are expected for use as resources for cell transplantation therapies for various diseases such as Parkinson's disease, juvenile diabetes, and leukemia, taking advantage of the aforementioned properties. However, transplantation of ES cells has a problem of causing rejection in the same manner as organ transplantation. Moreover, from an ethical viewpoint, there are many dissenting opinions against the use of ES cells which are established by destroying human embryos.
Embryonic stem (ES) cells, derived from the inner cell mass of mammalian blastocysts, have the ability to grow indefinitely while maintaining pluripotency (Evans et al., Nature 292:154-156, 1981; Martin, P.N.A.S. USA 78:7634-7638, 1981). These properties have led to expectations that human ES cells might be useful to understand disease mechanisms, to screen effective and safe drugs, and to treat patients of various diseases and injuries, such as juvenile diabetes and spinal cord injury (Thomson et al., Science 282:1145-1147, 1998). Use of human embryos, however, faces ethical controversies that hinder the applications of human ES cells. In addition, it is difficult to generate patient- or disease-specific ES cells, which are required for their effective application. Therefore, if dedifferentiation of a patient's own somatic cells could be induced to establish cells having pluripotency and growth ability similar to those of ES cells (in this specification, these cells are referred to as “induced pluripotent stem cells (iPS cells)”, though they are sometimes called “embryonic stem cell-like cells” or “ES-like cells”), it is anticipated that such cells could be used as ideal pluripotent cells, free from rejection or ethical difficulties.
Methods for nuclear reprogramming of a somatic cell nucleus have been reported. One technique for nuclear reprogramming which has been reported involves nuclear transfer into oocytes (Wakayama et al., Nature 394:369-374, 1998; Wilmut et al., Nature 385:810-813, 1997). Another method, for example, a technique of establishing an embryonic stem cell from a cloned embryo, prepared by transplanting a nucleus of a somatic cell into an egg, was reported (Hwang et al., Science 303:1669-74, 2004; Hwang et al., Science 308:1777-83, 2005): these articles were, however, proved to be fabrications and later withdrawn. Others have reported techniques for nuclear reprogramming of a somatic cell nucleus by fusing a somatic cell and an ES cell (Tada et al., Curr. Biol. 11:1553-1558, 2001; Cowan et al., Science 309:1369-73, 2005). Another reported technique for reprogramming a cell nucleus involves treatment of a differentiated cell with an undifferentiated human carcinoma cell extract (Taranger et al., Mol. Biol. Cell 16:5719-35, 2005). However, these methods all have serious drawbacks. Methods of nuclear transfer into oocytes and techniques which involve the fusion of ES and differentiated cells both comprise the use of ES cells, which present ethical problems. In addition, cells generated by such methods often lead to problems with rejection upon transplantation into an unmatched host. Furthermore, the use of cell extracts to treat differentiated cells is technically unreliable and unsafe, in part because the cell extract components responsible for the nuclear programming are mixed in solution with other unknown factors.
A method for screening a nuclear reprogramming factor having an action of reprogramming differentiated somatic cells to derive induced pluripotent stems cell was proposed in International Publication WO2005/80598, which is incorporated by reference in its entirety. This method comprises the steps of: contacting somatic cells containing a marker gene under expression regulatory control of an ECAT (ES cell associated transcript) gene expression control region with a test substance; examining presence or absence of the appearance of a cell that expresses the marker gene; and choosing a test substance inducing the appearance of said cell as a candidate nuclear reprogramming factor for somatic cells. A method for reprogramming a somatic cell is disclosed in Example 6 and the like of the above publication. However, this publication fails to report an actual identification of a nuclear reprogramming factor.
In view of these problems, there remains a need in the art for nuclear reprogramming factors capable of generating pluripotent stem cells from somatic cells. There also remains a need for pluripotent stem cells, which can be derived from a patient's own somatic cells, so as to render ethical issues and avoid problems with rejection. Such cells would have enormous potential for both research and clinical applications.